Electrode active material having core-shell structure

ABSTRACT

Disclosed is an electrode active material having a core-shell structure, which includes: (a) a core capable of intercalating and deintercalating lithium ions; and (b) a shell including a polymer or an oligomer having a glass transition temperature of 25° C. or less when impregnated with an electrolyte, wherein a surface of the core is coated with the shell. Also, an electrode manufactured by using the electrode active material and a secondary battery including the electrode are disclosed. The shell (b) suppresses the formation of an SEI layer during initial charge of a battery, and prevents initial capacity reduction.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application is a continuation of U.S. application Ser. No.12/670,798 (filed Feb. 1, 2011, and now issued as U.S. Pat. No.8,632,698), which is a national phase entry under 35 U.S.C. §371 ofInternational Application No. PCT/KR2008/004360, filed Jul. 25, 2008,published in English, which claims the benefit of Korean PatentApplication Nos. 10-2007-0075337, filed Jul. 26, 2007 and10-2007-0075339, filed Jul. 26, 2007. The disclosures of saidapplications are incorporated by reference herein.

TECHNICAL FIELD

The present invention relates to an electrode active material designedto improve the initial capacity of a battery, stabilize anelectrochemical property during charge/discharge, and maintain a highcapacity.

BACKGROUND ART

There has been recently increasing interest in energy storagetechnology. Electrochemical devices have been widely used as energysources in portable phones, camcorders, notebook computers, PCs andelectric cars, resulting in intensive research and development. In thisregard, electrochemical devices are subjects of great interest.Particularly, development of rechargeable secondary batteries has beenthe focus of attention. Also, in developing such batteries, research ondesign of a novel electrode and a novel battery has been recentlyconducted in order to improve capacity density and specific energy.

Among the currently used secondary batteries, lithium secondarybatteries, developed in early 1990's, generally have drive voltage andenergy density higher than those of conventional batteries using aqueouselectrolytes such as batteries, Ni—Cd batteries, H₂SO₄—Pb batteries,etc., and thus they are spotlighted in the field of secondary batteries.

A lithium secondary battery may be generally manufactured by using acathode and an anode, which include electrode active materials capableof intercalating/deintercalating lithium ions, and an electrolytefunctioning as conduction medium of the lithium ions. Meanwhile, thelithium secondary battery is rechargeable and dischargeable becauselithium ions coming out from a cathode active material during a chargeprocess are intercalated into an anode active material, anddeintercalated during a discharge process, so that the lithium ions runbetween both electrodes while serving to transfer energy.

However, in the lithium secondary battery, there is a problem in thatduring charge/discharge, side reactions occur within the battery bydecomposition of a nonaqueous electrolyte solution functioning as anelectrolyte, particularly, a carbonate organic solvent, on an electrodesurface. Also, when an electrolyte solvent having a large molecularweight, such as ethylene carbonate (EC), propylene carbonate (PC), etc.is co-intercalated between graphite layers of a carbon-based anode, thestructure of the anode may be broken down.

It has been known that the above mentioned problems can be solved by asolid electrolyte interface (SEI) layer formed on the anode surfaceduring initial charge of a battery, the SEI layer allowing lithium ionsto pass while functioning as a protective layer of the anode surface.

Meanwhile, it is assumed that the SEI layer is formed by reduction of anelectrolyte component, a reaction between an electrolyte and acarbon-based anode active material, etc. during initial charge, butlithium ions within a battery irreversibly participate in the formation,thereby reducing the initial capacity of the battery. Accordingly, it isdifficult to achieve a high capacity battery.

As an anode active material, a carbon material is mainly used. When acarbon material is used, a high voltage battery can be achieved due to alow potential vs. lithium potential, but it is difficult to achieve ahigh capacity battery due to a maximum theoretical capacity of onlyabout 370 mAh/g.

As an attempt to achieve a high capacity battery, a method ofsubstituting the carbon electrode active material by a metal or ametalloid-based active material having a high electric capacity, such asSi, has been researched. However, when the metal(loid)-based activematerial is used, the volume significantly changes according tointercalation/deintercalation of lithium ions, thereby causing problemsto be solved, such as cyclability degradation by decomposition of theactive material, battery stability degradation by gas generation in alarge amount during charge/discharge, etc.

DISCLOSURE Technical Problem

In the process of solving the problem of initial capacity reduction in asecondary battery, the inventors of the present invention inventedintroduction of an electrode active material having a core-shellstructure, instead of a conventional electrode active material, thecore-shell structure including (a) a core capable of intercalating anddeintercalating lithium ions, and (b) a shell including a specificpolymer or a specific oligomer.

More specifically, the inventors found that when a polymer or anoligomer (having a glass transition temperature of 25° C. or less whenimpregnated with an electrolyte) is used for the shell (b), the shell(b) can prevent initial capacity reduction by suppressing the formationof an SEI layer during initial charge of a battery, and can coverfunctions of a conventional SEI. Also, it has been known that the shell(b) can have sufficient elasticity in an operating temperature range ofa battery, and thus is not easily broken down by a core volume changecaused by charge/discharge of a battery, and thereby can contribute toimprovement of battery stability.

Also, the inventors found that in using a metal(loid) based activematerial, such as Si, as an electrode active material, when an electrodeactive material of core-shell structure including a shell (b) coated onthe surface of the metal(loid) based active material is used, gasgeneration during charge/discharge of a battery is minimized, therebysolving the problem of battery stability reduction caused by the use ofthe metal(loid) based active material. Herein, the shell includes apolymer or an oligomer having a glass transition temperature of 25° C.or less when impregnated with an electrolyte.

The present invention is based on this finding.

Technical Solution

In accordance with an aspect of the present invention, there is providedan electrode active material having a core-shell structure, whichincludes: (a) a core capable of intercalating and deintercalatinglithium ions; and (b) a shell including a polymer or an oligomer havinga glass transition temperature of 25° C. or less when impregnated withan electrolyte, wherein a surface of the core is coated with the shell.

Also, the present invention provides an electrode manufactured by usingthe electrode active material.

Also, the present invention provides a secondary battery including theelectrode.

Hereinafter, the present invention will be described in detail.

An electrode active material of the present invention is characterizedby having a core-shell structure in which, on the surface of a core (a)capable of intercalating and deintercalating lithium ions, a shell (b)including a polymer or an oligomer (having glass transition temperatureof 25° C. or less when impregnated with an electrolyte) is coated.

The core (a) can repeatedly intercalate/deintercalate lithium ions,which allows a battery to be charged/discharged. Also, the core (a)includes a metal(loid) having a high electric capacity, and thus it ispossible to achieve a high capacity battery.

In the present invention, the shell (b) is impregnated with anelectrolyte, and thus does not prevent the lithium ions from moving intothe core (a). Also, the shell (b) can suppress formation of an SEI layerduring initial charge, and thus prevent the initial capacity of thebattery from being reduced, and thereby maximize a battery capacity.Also, the shell (b) can minimize gas generation during charge/dischargeof the battery, thereby improving the stability of the battery.

In general, an electrolyte transfers lithium ions between a cathode andan anode during charge/discharge of a battery, and may form apassivation layer (SEI layer) on the surface of an anode active materialby reacting with lithium ions within the battery during initial charge.

However, in general, in operation of a secondary battery, the reductionof an electric capacity occurs during an initial cycle. It is assumedthat this is because the formation of an SEI layer during initial chargeof the battery consumes many lithium ions within an electrolyte.Especially, such initial capacity reduction is more problematic in acarbon-based electrode active material having a small theoreticalcapacity.

Also, the SET layer usually includes an inorganic material such as LiF,Li₂CO₃, etc., and thus cannot resist a change in volume of an electrodeactive material, caused by intercalation and deintercalation of lithiumions during charge/discharge of a battery. Accordingly, on the SEIlayer, a crack may occur, and through the crack, an anode activematerial and an electrolyte may continuously contact/react with eachother, thereby reducing battery performance. Specifically, during thereaction of the anode active material and the electrolyte, theelectrolyte may be decomposed, and thus may continuously generate gassuch as CO₂. Then, the gas may reduce the stability of the battery, andincrease the thickness of the battery, resulting in a problem in setssuch as cellular phones, notebook PCs, etc. Especially, such a problemis more significant when a metal(loid)-based electrode active material,whose volume is changeable up to 200-300% byintercalation/deintercalation of lithium ions, is used.

However, according to the present invention, the shell (b) suppressesdirect contact with the core (a) capable ofintercalating/deintercalating lithium ions and a non aqueouselectrolyte, and thus the formation of an SEI layer during initialcharge of a battery can be prevented. Accordingly, in the presentinvention, it is possible to prevent an initial capacity of the batteryfrom being reduced, and to maximize the capacity of an electrode activematerial.

Also, in the present invention, the shell (b) can cover the functions ofa conventional SEI layer because the shell is impregnated with anelectrolyte, thereby transferring lithium ions to the core (a), andprotects the core (a), thereby suppressing side reactions between thecore and the electrolyte. Although an SEI layer can be formed on thesurface of the core (a) in the Present invention, the shell (b)impregnated with an electrolyte protects the SET layer or blocks the SEIlayer from the outside, reducing the occurrence of a crack on the SEIlayer. Accordingly, in the present invention, it is possible to suppressa side reaction between an electrode active material and an electrolyte,and to minimize gas generation.

Meanwhile, during charge/discharge of a battery, the core (a)intercalates/deintercalates lithium ions, which may significantly causea change in volume. Accordingly, when a polymer or an oligomer whichlacks chain flexibility in an operating temperature range of the battery(for example, a polymer or an oligomer having a glass transitiontemperature of 25° C. or more when impregnated with an electrolyte) isused as a component for the shell (b), without consideration of such avolume change, the shell (b) cannot stand the volume change of the core,and thus a crack may occur. Thus, the cyclability and stability of thebattery may be reduced.

However, in the present invention, the shell (b) includes a polymer oran oligomer which has a glass transition temperature of 25° C. or lesswhen impregnated with an electrolyte, and thus can have sufficientelasticity in an operating temperature range of a battery. Accordingly,the shell (b) of the present invention, which functions as a protectivelayer for the core, is not easily broken down by a core volume changecaused by charge/discharge of a battery, and thereby can contribute toimprovement of battery stability.

In the present invention, a carbon number, a substitute, a monomer, etc.of a material for the shell (b) are not particularly limited, as long asthe material includes a polymer or an oligomer having a glass transitiontemperature of 25° C. or less when impregnated with an electrolyte.Herein, the electrolyte is a conventionally known in the art, and isapplied to a battery manufactured according to the present invention.

Also, although a battery is generally operated at about room temperature(25° C.), the polymer or the oligomer preferably has a glass transitiontemperature of −20° C. or less when impregnated with an electrolyte inconsideration of operation at a temperature lower than the roomtemperature.

Also, in the shell (b), the polymer or the oligomer preferably includesan ether (—O—) group. The ether group included in the polymer or theoligomer selectively conducts lithium ions by chemically bonding to anadjacent ether group and lithium ion, thereby facilitating the movementof the lithium ions to the core (a). Non-limiting examples of thepolymer or the oligomer including an ether group (—O—) includepolyethylene glycol, polyethylene oxide, polyethylene glycol methylether, polyethylene glycol dimethyl ether, poly propylene oxide,polyethylene, polypropylene, polyisobutylene, polyvinylidene chloride,etc. and the materials may be used alone or in combination.

Also, the thickness of the shell (b) is preferably within a range of0.001 μm to 1 μm. If the thickness is less than 0.001 μm, a batteryperformance improving effect is not significant, and if the thickness isgreater than 1 μm, the movement of lithium ions to the core (a) may beprevented.

Meanwhile, there is no limitation in the core that may be used in thepresent invention, as long as the core can intercalate or deintercalatelithium ions. It is possible to use, as the core, a material for aconventional electrode active material, preferably a material for ananode active material, which can be used for an electrode of aconventional secondary battery, preferably an anode.

Examples of the anode active material include a carbon material, such ascarbon, petroleum coke, activated carbon, graphite, carbon fiber, etc.,or a metal, oxide. Non-limiting examples of the metal oxide includeTiO₂, Li₄Ti₅O₁₂, etc., which have potential vs. lithium potential ofless than 2V. The materials may be used alone or in combination.

Also, there is no limitation in the core (a) that may be used in thepresent invention, as long as the core is a metal(loid) material thatcan intercalate/deintercalate lithium ions, and includes a metal or ametalloid.

Non-limiting examples of the metal(loid) material include (i) a metal ora metalloid selected from the group including Si, Al, Sn, Sb, Bi, As,Ge, Pb, and Li; (ii) an oxide of the metal or the metalloid selectedfrom the group (i); an alloy of at least two metals or metalloidsselected from the group (i); (iv) a composite of a carbon material withthe metal or the metalloid selected from the group (i); or (v) acomposite of a carbon material with the oxide of the metal or themetalloid selected from the group (i), etc. and the materials may beused alone or in combination.

An electrode active material of the present invention may be prepared bya polymer coating method conventionally known in the art, and anembodiment of the method may include the steps of: i) adding a coreforming material in dispersion including a polymer or an oligomer (forforming a shell) and a solvent; stirring the solution obtained from stepi); and separating an electrode active material having a core-shellstructure from the solution obtained from step ii).

In step i), the weight ratio of the polymer or the oligomer (for forminga shell) to the core forming material ranges from 0.01:99.99 to 10:90,and the materials are preferably included in an amount of 0.01 to 10parts by weight, respectively, with respect to 100 parts by weight of asolution. If any one of the polymer or the oligomer (for forming ashell) and the core forming material is included in an excessive amount,it is difficult to thinly and uniformly form the shell on the surface ofthe core.

Also, the solvent in step i) is not particularly limited as long as thesolvent can be used in a conventional compound preparation process.Non-limiting examples of the solvent include a hydrophilic solvent, suchas water, methyl alcohol, ethyl alcohol, isopropyl alcohol, etc., anorganic polar solvent, such as methylene chloride, acetonitrile, ethylacetate, methyl ethyl ketone, chloroform, chlorobenzene,dimethylacetamide, dimethyl formamide, dimethyl sulioxide, etc., andorganic non-polar solvent, such as pentane, hexane, heptane, diethylether, toluene, benzene, xylene, cyclohexane, cyclopentane, carbontetrachloride, tetrahydrofuran, etc. The materials may be used alone orin combination.

The stirring in step is preferably carried out for 1 minute to 12 hours.If the stirring time is too short, it is difficult to form the shell onthe surface of the core, and also stirring for an excessive time is notadvantageous in shell formation.

The separating in step iii) may be carried out by a methodconventionally known to a skilled person in the art, for example, acentrifugal process.

Also, the present invention provides an electrode manufactured by usingan electrode active material having the core-shell structure, preferablyan anode.

The electrode according to the present invention may be manufactured bya conventional method known in the art, except that a core-shellelectrode active material prepared according to the present invention isused. In one embodiment of such conventional methods, slurry is preparedby mixing and agitating the core-shell electrode active material and asolvent, optionally with a binder, a conductive agent and a dispersant,and then the slurry is applied (coated) onto a metallic currentcollector, followed by compressing and drying. Herein, the weight ratioof the binder to the electrode active material may range from 1 to 10,and the weight ratio of the conductive agent to the electrode activematerial may range from 1 to 30.

The metallic current collector includes a metal with high conductivity.Any metal to which the electrode active material slurry can be adheredwith ease can be used as long as it shows no reactivity in the drivevoltage range of a battery using the same. Non-limiting examples of thecathode current collector may include a foil made of aluminum, nickel,or a combination thereof. And non-limiting examples of the anode currentcollector may include a foil made of copper, gold, nickel, copper alloy,or a combination thereof.

Examples of the binder that may be used in the present invention includepolytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVdF), etc.

There is no particular limitation in the conductive agent, as long asthe conductive agent is an electron conductive material that causes nochemical change in a secondary battery. In general, the conductive agentthat may be used in the present invention includes carbon black,graphite, carbon fibers, carbon nanotubes, metal powder, conductivemetal oxides, organic conductive agents, or the like. Commerciallyavailable conductive agents include acetylene black-based conductiveagents (available from Chevron Chemical Company or Gulf Oil Company),Ketjen Black EC series (available from Armak Company), Vulcan XC-72(available from Cabot Company) and Super P (available from MMM Co.).

Non-examples of the solvent in manufacture of the electrode include anorganic solvent, such as N-methyl-2-pyrrolidone (NMP), dimethylformamide (DMF), acetone, dimethylacetamide, etc., or water, and thesolvents may be used alone or in combination. The solvent is used in anamount enough to dissolve and disperse the electrode active material,the binder, and the conductive agent, in consideration of the thicknessof the applied slurry, and the yield.

Furthermore, the present invention provides a secondary battery whichincludes the electrode according to the present invention, preferably asecondary battery which includes a cathode, an anode manufacturedaccording to the present invention, a separator, and an electrolyte.

Non-limiting examples of the secondary battery include a lithium metalsecondary battery, a lithium ion secondary battery, a lithium polymersecondary battery, or a lithium ion polymer secondary battery.

The cathode that may be employed in the secondary batter of the presentinvention is not particularly limited, and may be manufactured byadhering a cathode active material to a cathode current collector,according to a conventional method known in the art. The cathode activematerial may include a conventional cathode active material that may beused in a cathode of a conventional secondary battery, and non-limitingexamples of the cathode active material may include, a lithiumtransition metal composite oxide such as LiM_(x)C_(y) (M=Co, Ni, Mn,Co_(a)Ni_(b)Mn_(c)) (for example, lithium manganese composite oxide suchas LiMn₂O₄, lithium nickel oxide such as LiNiO₂, lithium cobalt oxidesuch as LiCoO₂, lithium iron oxide, oxides substituted with othertransition metals, lithium containing vanadium oxide, etc), chalcogenide(for example, manganese dioxide, titanium disulfide, molybdenumdisulfide, etc.), etc. Preferably, the examples include LiCoO₂, LiNiO₂,LiMnO₂, LiMn₂O₄, Li(Ni_(a)Co_(b)Mn_(c))O₂(0<b<1, 0<c<1, a+b+c=1),LiNi_(1-Y)Co_(Y)O₂, LiCo_(1-Y)Mn_(Y)O₂, LiNi_(1-Y)Mn_(Y)O₂ (providedthat, 0≦Y<1), Li(Ni_(a)Co_(b)Mn_(c))O₄ (0<a<2, 0<b<2, 0<c<2, a+b+c=2)LiNn_(2-z)Ni_(z)O₄, LiMn_(2-z)Co_(z)O₄ (provided that, 0<Z<2), LiCoPO₄,LiFePO₄, or a mixture thereof. Non-limiting examples of the cathodecurrent collector include foil, etc., obtained from aluminum, nickel, ora combination thereof.

The electrolyte is a conventional electrolyte known in the art, and mayinclude an electrolyte salt, and an electrolyte solvent. There is noparticular limitation in the electrolyte salt, as long as theelectrolyte salt is generally used for a non-aqueous electrolyte.Non-limiting examples of an electrolyte salt, which can be used in thepresent invention, include salts having a structure such as A⁺B⁻,wherein A⁺ contains an ion selected from among alkaline metal cations,such as Li⁺, Na⁺ and K⁺, and combinations thereof, and B⁻ contains anion selected from among anions, such as PF₆ ⁻, BF₄ ⁻, Cl⁻, Br⁻, I⁻, ClO₄⁻, AsF₆ ⁻, CH₃CO₂ ⁻, CF₃SO₃ ⁻, N(CF₃SO₂)₂ ⁻, and C(CF₂SO₂)₃ ⁻, andcombinations thereof. Particularly, a lithium salt is preferred. Theelectrolyte salts may be used alone or in combination.

There is no particular limitation in the electrolyte solvent, as long asthe electrolyte solvent is generally used as an organic solvent for anon aqueous electrolyte, and examples of the electrolyte solvent mayinclude a cyclic carbonate, a linear carbonate, lactone, ether, ester,sulfoxide, acetonitrile, lactam, and/or ketone. Examples of the cycliccarbonate include ethylene carbonate (EC), propylene carbonate (PC),butylene carbonate (BC), fluoroethylene carbonate (FEC), etc.; andexamples of the linear carbonate include diethyl carbonate (DEC),dimethyl carbonate (DMC), dipropyl carbonate (DPC), ethyl methylcarbonate (EMC), methyl propyl carbonate (MPC), etc. Examples of thelactone include gamma-butyrolactone (GBL); and examples of the etherinclude dibutyl ether, tetrahydrofuran, 2-methyltetrahydrofuran,1,4-dioxane, 1,2-dimethoxyethane, 1,2-diethoxyethane, etc. Examples ofthe ester include methyl formate, ethyl formate, propyl formate, methylacetate, ethyl acetate, propyl acetate, methyl propionate, ethylpropionate, butyl propionate, methyl pivalate, etc. Also the sulfoxidemay include dimethyl sulfoxide; the lactam may includeN-methyl-2-pyrrolidone (NMP); and the ketone may include polymethylvinylketone. Also, a halogen derivative of the organic solvent may be used.The organic solvents may be used alone or in combination.

A separator which can be used in the present invention is not limited toany specific separator, but a porous separator is preferred, andnon-limiting examples thereof include porous polypropylene, polyethyleneor polyolefin separators.

The secondary battery according to the present invention may be obtainedaccording to a conventional method known in the art. For example, thebattery may be obtained by interposing a separator between an anode anda cathode and injecting an electrolyte thereto.

There is no particular limitation in the outer shape of the secondarybattery according to the present invention. The secondary battery may bea cylindrical battery using a can, a prismatic battery, a pouch-typebattery, or a coin-type battery.

Advantageous Effects

Since an electrode active material having a core-shell structureaccording to the present invention uses, as a component for the shell, apolymer or an oligomer having a glass transition temperature of 25° C.or less when impregnated with an electrolyte, it is possible to preventinitial capacity reduction by suppressing the formation of an SEI layerduring initial charge of a battery, and to maximize a capacity of anelectrode active material.

Also, in the present invention, the shell can have sufficient elasticityin an operating temperature range of a battery, and thus is not easilybroken down by a core volume change caused by charge/discharge of abattery, and thereby can contribute to improvement of battery stability.

Also, the electrode active material having a core-shell structureaccording to the present invention, especially a metal(loid) basedelectrode active material having a core-shell structure can have aslightly higher electric capacity, compared to a conventional electrodeactive material, especially a conventional metal (loid) based electrodeactive material, and can minimize gas generation duringcharge/discharge. Accordingly, through the present invention, it ispossible to achieve a high capacity battery, and to solve stabilityreduction caused by the use of a conventional metal(loid) basedelectrode active material.

DESCRIPTION OF DRAWINGS

FIG. 1 is a graph showing a discharge capacity maintenance ratio withrespect to cycles, which was measured in Experimental Example 2;

FIG. 2 is a graph showing charge/discharge efficiency with respect tocycles, which was measured in Experimental Example 2;

FIG. 3 is a graph showing a discharge capacity maintenance ratio withrespect to cycles, which was measured in Experimental Example 3;

FIG. 4 is a graph showing charge/discharge efficiency with respect tocycles, which was measured in Experimental Example 3;

FIG. 5 is a graph showing a discharge capacity maintenance ratio withrespect to cycles, which was measured in Experimental Example 4; and

FIG. 6 is a graph showing charge/discharge efficiency with respect tocycles, which was measured in Experimental Example 4.

BEST MODE

Reference will now be made in detail to the preferred embodiments of thepresent invention. However, the following examples are illustrativeonly, and the scope of the present invention is not limited thereto.

Example 1 1-1. Preparation of an Electrode Active Material

0.1 parts by weight of polyethylene glycol methyl ether was dissolved in100 parts by weight of ethyl alcohol, and graphite powder was added tothe solution in an amount of 10 parts by weight per 100 parts by weight:of the solution, followed by stirring for 12 hours. Then, the mixedsolution was filtered to obtain a core-shell structure electrode activematerial including polyethylene glycol methyl ether coated on graphitepowder with a thickness of about 25 nm.

1-2. Preparation of an Electrode

The electrode active material obtained from Example 1-1, SBR(styrenebutadiene rubber), and CMC(carboxy methyl cellulose) were mixed in aweight ratio of 96:2:2, and water was added thereto to prepare slurry.The prepared slurry was applied to copper foil with a thickness of 10μm, and a drying process was carried to obtain an electrode, followed byroll-press.

1-3. Manufacture of a Battery

The electrode obtained from Example 1-2, and a lithium metal thin filmas a counter electrode were used, a polyethylene separator wasinterposed between the anode and the cathode, and an electrolyte wasinjected thereto to obtain a secondary battery. Herein, as theelectrolyte, 1M LiPF₆ solution including EC/EMC (ethyl carbonate:diethyl carbonate (DEC)=1:2 in a volume ratio) was used.

Comparative Example 1

A secondary battery was obtained in the same manner as described inExample 1-3, except that an electrode including conventional graphitepowder (which is not coated with polyethylene glycol methyl ether) as anelectrode active material was used instead of the electrode obtainedfrom Example 1-2.

Experimental Example 1 Measurement of Glass Transition Temperature

The glass transition temperature was measured by using DSC (DifferentialScanning calorimeter), which was carried out by impregnatingpolyethylene glycol methyl ether used in Example 1-1 with anelectrolyte, and raising temperature from −100° C. to 150° C. at a rateof 5° C. per minute. Herein, as the electrolyte, the same 1M LiPF₆solution including EC/EMC (ethyl carbonate: diethyl carbonate (DEC)=1:2in a volume ratio) as that of Example 1-3 was used.

As a result, the measured value of the glass transition temperature ofpolyethylene glycol methyl ether was −56° C.

Experimental Example 2 Test on Battery Performance

Each of the secondary batteries obtained from Example 1 and ComparativeExample 1 was charged at a rate of 0.1 C up to 5 mV and charged to acurrent of 0.005 C at 5 mV, and then was discharged to 1V at a rate of0.1 C, at 25° C. This charge/discharge was carried out twice. Then,charge/discharge was carried out at 0.5 C/0.5 C in the same manner asdescribed above, and the discharge capacity maintenance ratio andcharge/discharge efficiency according to a charge/discharge cycle weremeasured. Table 1 and FIGS. 1 and 2 show the results.

TABLE 1 Exp. 1 Comp. Exp. 1 Charge/discharge 1^(st). Eff. (%) 94.3 93efficiency 50^(th). Eff. (%) 100 100

As a result, compared to the battery obtained from Comparative Example1, which used a conventional anode active material (graphite powder),the battery obtained from Example 1, which used the core-shell structureanode active material according to the present invention, showed highercharge/discharge efficiency during an initial charge/discharge cycle.Then, during following cycles, an initial capacity and charge/dischargeefficiency of the battery obtained from Example 1 was similar to thebattery obtained from Comparative Example 1 (see FIGS. 1 and 2).

Accordingly, it can be seen from the results that when the core-shellstructure anode active material according to the present invention isused, formation of an SEI layer is suppressed during initial charge of abattery, thereby pro venting decrease of an initial capacity. Also, itis determined that the movement of lithium ions is not prevented by ashell, and thus battery performance is not degraded.

Example 2 2-1. Preparation of an Electrode Active Material

0.1 parts by weight of polyethylene glycol methyl ether was dissolved in100 parts by weight of ethyl alcohol, and a Si/graphite compositeparticle (diameter: 20 μm) was added to the solution in an amount of 10parts by weight per 100 parts by weight of the solution, followed bystirring for 12 hours, Then, the mixed solution was filtered to obtain acore-shell structure Si/graphite composite electrode active materialcoated with polyethylene glycol methyl ether with a thickness of about25 nm.

A core-shell structure graphite-based electrode active material wasprepared in the same manner as described in Example 2-1, except thatgraphite powder was used, instead of the Si/graphite composite particle.

2-2. Preparation of an Electrode

The core-shell structure Si/graphite composite electrode active materialobtained from Example 2-1, the core-shell structure graphite-basedelectrode active material, the SBR(styrene butadiene rubber), andCMC(carboxy methyl cellulose) were mixed in a weight ratio of4.8:91.2:2:2, and water was added thereto to prepare slurry. Theprepared slurry was applied to copper foil with a thickness of 10 μm,and a drying process was carried to obtain an electrode, followed byroll-press.

2-3. Manufacture of a Battery

The electrode obtained from Example 2-2, and a lithium metal thin filmas a counter electrode were used, a polyethylene separator wasinterposed between both electrodes, and an electrolyte was injectedthereto to obtain a secondary battery. Herein, as the electrolyte, 1MLiPF₆ solution including EC/EMC (ethyl carbonate: diethyl carbonate(DEC)=1:2 in a volume ratio) was used.

Example 3

An electrode active material, an electrode, and a secondary battery weremanufactured in the same manner as described in Example 2, except thatpolyethylene glycol methyl ether was used in an amount of 0.5 parts byweight with respect to 100 parts by weight of ethyl alcohol, to preparea core-shell structure Si/graphite composite electrode active materialcoated with the polyethylene glycol methyl ether with a thickness ofabout 43 nm.

Comparative Example 2

A secondary battery was manufactured in the same manner as described inExample 2-3, except that in preparing an electrode, a conventionalSi/graphite composite particle was used as an electrode active material,instead of the core-shell structure Si/graphite composite electrodeactive material and the core-shell structure graphite-based electrodeactive material obtained from Example 2-1.

Experimental Example 3 Test on Battery Performance

Each of the secondary batteries obtained from Examples 2 and 3, andComparative Example 2 was charged at a rate of 0.1 C up to 5 mV, andcharged to a current of 0.005 C at 5 mV, and then was discharged to 1Vat a rate of 0.1 C, at 25° C. This charge/discharge was carried outtwice. Then, charge/discharge was carried out at 0.5 C/0.5 C in the samemanner as described above. Herein, after 50 cycles, an increase ratio ofthickness of the battery was measured, and the results were noted inTable 2. Also, the discharge capacity maintenance ratio andcharge/discharge efficiency according to charge/discharge cycle weremeasured. Table 3 and FIGS. 3 and 4 show the results.

TABLE 2 Electrode Thickness Initial electrode thickness after increaseratio after thickness (μm) 50 cycles (μm) 50 cycles (%) Exp. 2 66 8626.5 Exp. 3 68 80 18.2 Comparative 62 90 45.6 Exp. 2

TABLE 3 1^(st). 1^(st). Charge 1^(st). Discharge Charge/dischargecapacity (mA) capacity (mA) efficiency (%) Exp. 2 452.7 414.8 91.3 Exp.3 454.8 415.8 91.5 Comparative 465.5 419.1 90.1 Exp. 2

As a result, each of the batteries obtained from Examples 2 and 3, whichused the core-shell structure Si/graphite composite electrode activematerial according to the present invention, showed an electrodethickness increase ratio 1.7˜2.5 times lower compared to the batteryobtained from Comparative Example 2, which used a conventionalSi/graphite composite electrode active material (see Table 2). Also, theinitial charge/discharge efficiency of the battery obtained from Example2 was slightly higher than that of the battery obtained from ComparativeExample 2 (see Table 3).

Accordingly, it can be seen from the results that the core-shellstructure metal(loid) anode active material according to the presentinvention can show an electricity capacity higher than a conventionalmetal(loid) electrode active material while improving the stability of abattery by significantly reducing gas generation duringcharge/discharge.

Meanwhile, as shown in FIGS. 3 and 4, the battery according to thepresent invention (obtained from Example 2) showed similar performanceto the battery obtained from Comparative Example 2, from the standpointof cyclability and charge/discharge efficiency. Accordingly, it isdetermined, that introduction of the core-shell structure electrodeactive material (especially, shell) according to the present inventiondoes not cause degradation of battery performance.

Example 4 4-1. Preparation of an Electrode Active Material

0.1 parts by weight of polyethylene glycol methyl ether was dissolved in100 parts by weight of ethyl alcohol, and a SiC/graphite compositeparticle (diameter: 20 μm) was added to the solution in an amount of 10parts by weight per 100 parts by weight of the solution, followed bystirring for 12 hours. Then, the mixed solution was filtered to obtain acore-shell structure SiO/graphite composite electrode active materialcoated with polyethylene glycol methyl ether with a thickness of about25 nm.

A core-shell structure graphite-based electrode active material wasprepared in the same manner as described in Example 4-1, except thatgraphite powder was used, instead of the SiO/graphite compositeparticle.

4-2. Preparation of an Electrode

The core-shell structure SiO/graphite composite electrode activematerial obtained from Example 4-1, the core-shell structuregraphite-based electrode active material, SBR(styrene butadiene rubber),and CMC(carboxy methyl cellulose) were mixed in a weight ratio of14.6:82.4:1.5:1.5, and water was added thereto to prepare slurry. Theprepared slurry was applied to copper foil with a thickness of 10 μm,and a drying process was carried to obtain an electrode, followed byroll-press.

4-3. Manufacture of a Battery

A secondary battery was manufactured in the same manner as described inExample 2-3, except that the electrode obtained from Example 4-2 wasused instead of the electrode obtained from Example 2-2.

Comparative Example 3

A secondary battery was manufactured in the same manner as described inExample 4-3, except that in preparing an electrode, a conventionalSb/graphite composite particle was used as an electrode active materialinstead of the core-shell structure SiO/graphite composite electrodeactive material and the core-shell structure graphite-based electrodeactive material obtained from Example 4-1.

Experimental Example 4 Test on Battery Performance

Each of the secondary batteries obtained from Example 4, and ComparativeExample 3 was charged at a rate of 0.1 C up to 5 mV, and charged to acurrent of 0.0050 at 5 mV, and then was discharged to 1.5V at a rate of0.10, at 25° C. This charge/discharge was carried out twice. Then,charge/discharge was carried out at 0.5 C/0.5 C in the same manner asdescribed above. Herein, after 50 cycles, an increase ratio of thicknessof the battery was measured, and the results were noted in Table 4.Also, the discharge capacity maintenance ratio and charge/dischargeefficiency according to a charge/discharge cycle were measured. Table 5and FIGS. 5 and 6 show the results.

TABLE 4 Electrode Thickness Initial electrode thickness after increaseratio after thickness (μm) 50 cycles (μm) 50 cycles (%) Exp. 4 65 8031.1 Comparative 64 83 41.0 Exp. 3

TABLE 5 1^(st). 1^(st). Charge 1^(st). Discharge Charge/dischargecapacity (mAh/g) capacity (mAh/g) efficiency (%) Exp. 4 661.5 535.8 81.0Comparative 666.8 536.2 80.3 Exp. 3

As a result, each of the batteries obtained from Example 4, which usedthe core-shell structure SiO/graphite composite electrode activematerial according to the present invention, showed an electrodethickness increase ratio about 1.3 times lower compared to the batteryobtained from Comparative Example 3, which used a conventional.SiO/graphite composite electrode active material (see Table 4). Also,the initial charge/discharge efficiency of the battery obtained fromExample 4 was slightly higher than that of the battery obtained fromComparative Example 3 (see Table 5).

Meanwhile, as shown in FIGS. 5 and 6, the battery according to thepresent invention (obtained from Example 4) showed similar performanceto the battery obtained from Comparative Example 3, from the standpointof cyclability and charge/discharge efficiency. Accordingly, it isdetermined that introduction of metal oxide-based anode active material,such as the core-shell structure electrode active material (especially,shell) according to the present invention does not cause degradation ofbattery performance.

The invention claimed is:
 1. An anode active material having acore-shell structure, which comprises (a) a core capable ofintercalating and deintercalating lithium ions; and (b) a shellcomprising a polymer or an oligomer having a glass transitiontemperature of 25° C. or less when impregnated with an electrolyte,wherein a surface of the core is coated with the shell, wherein the core(a) comprises (i) a metal or a metalloid selected from the groupincluding Si, Al, Sn, Sb, Bi, As, Ge, and Pb; (ii) an oxide of the metalor metalloid selected from the group (i); (iii) an alloy of at least twometals or metalloids selected from the group (i); (iv) a composite of acarbon material with the metal or metalloid selected from the group (i);or (v) a composite of a carbon material with the oxide of the metal ormetalloid selected from the group (i), wherein the shell (b) comprisesthe polymer or oligomer selected from the group consisting ofpolyethylene glycol, polyethylene oxide, polyethylene glycol methylether, polyethylene glycol dimethyl ether, poly propylene oxide,polyethylene, polypropylene, polyisobutylene, and polyvinylidenechloride, and wherein the shell (b) has a thickness ranging from 0.001μm to 1 μm.
 2. The electrode active material as claimed in claim 1,wherein the shell (b) comprises the polymer or oligomer having a glasstransition temperature of −20° C. or less when impregnated with anelectrolyte.
 3. An anode comprising the electrode active material asclaimed in claim
 1. 4. A secondary battery comprising the anode asclaimed in claim 3.